Javascript/Python Array Quick Reference

Whenever I’m learning or using an unfamiliar programming language, I often find that even some common tasks can become overwhelming. A quick reference table that refers back to my familiar programming language often helps me carry over tricks that I already know, at least initially.

The tables below are intended to help JavaScript/Python programmers learn and quickly find idiomatic ways to perform certain tasks on lists/arrays in Python/JavaScript.

Throughout this post:

f refers to a function.
a and a1 refer to a JavaScript array or a Python list.
i, begin, end are integers that refer to some element indices.
e and eN refer to an element.

Accessing

JavaScript	Python
`a[i]`	`a[i]`
`a.at(-i)`	`a[-i]`
`a.slice()`	`a[:]`
`a.slice(begin)`	`a[begin:]`
`a.slice(0, end)`	`a[:end]`
`a.slice(begin, end)`	`a[begin:end]`

Iterating

JavaScript	Python
`a.forEach(f) // f(e)`	`for e in a: f(e)`
`a.forEach(f) // f(e, i)`	`for i, e in enumerate(a): f(e, i)`

Mapping

JavaScript	Python
`a.map(f) // f(e)`	`[f(e) for e in a]` or `list(map(f, a))`
`a.map(f) // f(e, i)`	`[f(e, i) for i, e in enumerate(a)]`

Adding Elements

JavaScript	Python
`a.push(e)`	`a.append(e)`
`a.unshift(e)`	`a.insert(0, e)`
`a.splice(i, 0, e)`	`a.insert(i, e)`
`a.concat(a1)`	`a + a1`
`a = a.concat(a1)`	`a.extend(a1)`

Removing Elements

JavaScript	Python
`a.pop()`	`a.pop()`
`a.shift()`	`a.pop(0)`
`a.splice(i, 1)`	`a.pop(i)` or `del a[i]`
`a.splice(i, n)`	`del a[i:i+n]`

Modifying Elements

JavaScript	Python
`a[i] = e`	`a[i] = e`
`a.splice(i, 3, e0, e1, e2)`	`a[i:i+3] = [e0, e1, e2]`

Filtering

JavaScript	Python
`a.filter(f) // f(e)`	`[e for e in a if f(e)]` or `list(filter(f, a))`
`a.filter(f) // f(e, i)`	`[e for i, e in enumerate(a) if f(e, i)]`

Lookup

JavaScript	Python
`a.find(f)`	`next(filter(f, a))` or `next(e for e in a if f(e))`
`a.findLast(f)`	`next(e for e in reversed(a) if f(e))`
`a.indexOf(e)`	`a.index(e)`
`a.indexOf(e, begin)`	`a.index(e, begin)`
`a.slice(begin, end).indexOf(e) + begin`	`a.index(e, begin, end)`

Inclusion/Exclusion

JavaScript	Python
`a.includes(e)`	`e in a`
`a.some(f)`	`any(f(e) for e in a)`
`a.every(f)`	`all(f(e) for e in a)`

String

JavaScript	Python
`a.join(s) // s is a string`	`s.join(a)`
`a.toString()`	`','.join(a)`

Misc

JavaScript	Python
`a.flat()`	`list(itertools.chain.from_iterable(a))` if `a` is an iterable of lists
`a.reverse()`	`a.reverse()`
`a.sort()`	`a.sort()`
`a.sort(f)`	`a.sort(key=functools.cmp_to_key(f))`
`a.sort((a, b) => (f(a) - f(b)))`	`a.sort(key=f) # f returns a number`

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Accessing#

Iterating#

Mapping#

Adding Elements#

Removing Elements#

Modifying Elements#

Filtering#

Lookup#

Inclusion/Exclusion#

String#

Misc#